Dry-wet variations and influencing factors in Northeast China based on evapotranspiration

Dry–wet variation is one of the most important indicators to reflect the surface water supply–demand balance under various climates, leading directly to hydrological processes, agricultural stability, and ecosystem security. Particularly, Northeast China, as the grain production base and ecological barrier, can dominate the national food security and sustainable development. Therefore, it is often required to explore the driving mechanisms of dry–wet variations in this region. This study aims to determine the spatiotemporal variations in evapotranspiration and dry–wet conditions in Northeast China from 2001 to 2024. A systematic analysis was also conducted to further explore the driving factors using multiple influencing variables. MOD16A2GF evapotranspiration dataset was constructed for the regional dry–wet variations. Evapotranspiration observations were collected from two stations, and meteorological data were from 116 stations. A linear regression method was applied to correct actual evapotranspiration (ET) and potential evapotranspiration (PET). A leave-one-year-out cross-validation was used to evaluate the accuracy and reliability of the parameters. Furthermore, the crop water stress index (CWSI) was calculated for surface dry–wet conditions using ET and PET after correction. The results showed that: (1) MOD16A2GF data shared the better agreement with observed values, in terms of variation trends. Compared with the original data, the root mean square error (RMSE) and Bias were reduced, while the Nash–Sutcliffe efficiency (NSE) was improved, indicating the high data accuracy and reliability. (2) ET values showed a significantly increasing trend from 2001 to 2024, with a growth rate of 3.11 mm/a. Meanwhile, PET and CWSI values shared a significantly decreasing trend, with an annual decline rate of 3.23 mm/a and 0.01, respectively, indicating an overall wetting trend and a gradual alleviation of water stress. (3) In terms of spatial patterns, ET decreased from 100 to 800 mm from east to west. PET and CWS decreased from 500 to 1,200 mm and from 0.1 to 0.9, respectively, from southwest to northeast. The maximum CWSI values, representing the most severe water stress, were observed in the southeastern part of the Eastern Four Leagues in Inner Mongolia. From the perspective of spatial trends, most regions showed increasing ET and decreasing PET, indicating the widespread wetting trend in the study area. Moreover, there were largely overlapped areas with a significant decrease in CWSI, where ET significantly increased, while PET decreased, further confirming the spatial consistency and robustness of the wetting trend. (4) Precipitation was positively correlated with ET and negatively correlated with PET and CWSI. The increase in precipitation significantly promoted actual evapotranspiration and suppressed potential evapotranspiration demand, thus dominating the dry–wet variations in Northeast China. In addition, agricultural irrigation increased surface water supply, while vegetation growth regulated the water exchange process between the land surface and the atmosphere. Both driving factors enhanced actual evapotranspiration for less water stress. The regional water resource can be allocated to prevent drought risks in sustainable agriculture under climatic conditions.